Before spray coating a surface, one has to ensure that the surface is free of dirt or residual coating, as these could impair adhesion of the coating to the substrate. In some cases, it is difficult to visually detect the presence of a residual coating. The residual coating may have the same appearance that the surface it is partially covering. In such cases, one would unknown to him/her spray coat a surface that is covered with residual coating and therefore obtain a coating of lesser quality.